1. Field of the Invention
The present invention relates to a method and device for forming images by nuclear magnetic resonance of the type used in the medical field.
Image formation by nuclear magnetic resonance is becoming more and more widespread mainly as a medical diagnostic means. By this means internal tissue structures can be made visible to the eye with a contrast and resolution never attained simultaneously by the other image forming methods. In order to obtain an image by nuclear magnetic resonance (MRI)of an organ section with differentiation of the characteristics of the tissue of this organ, the property is used which is possessed by certain atomic nuclei such as protons of orienting their magnetic moments, associated with the spins of the nucleus, while acquiring energy when they are placed in a main magnetic field B.sub.0. A particular zone of the object containing nuclei then has an overall magnetic moment which can be caused to swing in a given orientation, perpendicular to or coaxial with the field B.sub.0, while inducing a resonance by emission of a radio frequency field perpendicular to the main field.
All the nuclei then having a magnetic moment rotating at a precession frequency, called Larmor's frequency, tend to return to the initial orientation due to B.sub.0 while emitting a radio frequency signal at the characteristic resonance frequency of B.sub.0 and of the nucleus. This signal may be picked up by a reception antenna. The time required by the overall magnetic moment of a region considered to return to balanced conditions depends on two important factors: the spin-mesh work interaction and the spin-spin interaction of the nuclei with the surrounding matter. These two factors lead to the definition of two relaxation times called T.sub.1 and T.sub.2. A region considered of an object emits then a signal whose intensity depends on T.sub.1, T.sub.2, on the protonic density in the region and on the time which has passed since the radio frequency excitation.
In order to locate a region of the organ it is necessary to identify the nature of its emission as a function of the local magnetic field conditions. These local conditions are imposed so that the emission frequency is characteristic of the location in space of the region of the organ. For that, pulsed magnetic field gradients are superimposed on the main field B.sub.0. These gradients are oriented in directions X, Y, Z for defining at all times volume elements which emit signals at known precession frequencies. For acquiring an image, the local conditions are imposed by programmed sequences. These are stored in a pilot computer. These sequences also define the excitation times of the nuclei by the so called selective radio frequency pulses as well as the reading or acquisition times of the data of the image.
A description of different sequences generally used may be found in the article by W. A. Edelstein et al. Journal of Computer Assisted Tomography 7 (3) 391.401.1983. This article which reviews the respective performances, taking into consideration the signal to noise ratio of the image data acquired, of the different types of sequence, further determines that from this point of view the most favorable for discriminating the relaxation time T.sub.1 is the so called overlapping-saturation sequence.